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What Is the BMI Formula? Calculate BMI Step by Step
What is the BMI formula? Learn to calculate BMI step by step with metric and imperial examples, mistake-proof tips, and what your result means.
What is the BMI formula?
The BMI formula is body mass divided by height squared: in metric units, BMI = weight (kg) / height (m)²; in imperial, BMI = 703 × weight (lb) / height (in)². Measure your weight and height, convert height to meters (or inches), square the height, and divide. For example, 70 kg at 1.75 m gives 70 / 3.0625 = 22.9 — a healthy result.
One formula, three steps, thirty seconds. Let's walk through it.
The BMI formula in its simplest form
Body Mass Index is weight divided by height squared. In metric units that is kilograms divided by meters squared; in imperial it is 703 times pounds divided by inches squared. The BMI formula asks nothing about age, muscle, or lifestyle — just two measurements. Everything that follows is about getting those two measurements right.
The BMI formula is simple on purpose. The care goes into the inputs.
Step by step: the metric calculation
First, measure your height in metres. If you know it in centimetres, divide by 100: 175 cm becomes 1.75 m. Second, square the height: 1.75 × 1.75 = 3.0625. Third, divide your weight in kilograms by that number: 70 / 3.0625 = 22.86, so BMI ≈ 22.9. Three steps, in that order — convert, square, divide. Never skip the squaring; the BMI formula is meaningless without it.
Convert, square, divide. Say it like a mantra.
Step by step: the imperial calculation
If you work in pounds and inches, the BMI formula adds one conversion constant: BMI = 703 × lb / in². First, convert your height to total inches (5 ft 9 in = 69 in). Second, square it: 69 × 69 = 4,761. Third, multiply your weight by 703 and divide: 703 × 154 / 4,761 = 22.7. The 703 keeps the imperial BMI formula on the same scale as the metric one.
Same steps, one extra constant. Same answer.
Worked examples at different sizes
Three quick examples show the BMI formula in action. A 1.60 m person weighing 55 kg: 1.60² = 2.56, and 55 / 2.56 = 21.48 ≈ 21.5 — healthy. A 1.80 m person weighing 95 kg: 1.80² = 3.24, and 95 / 3.24 = 29.3 — overweight. A 5 ft 6 in, 180 lb person: 66² = 4,356, and 703 × 180 / 4,356 = 29.0 — overweight. The BMI formula handles every size the same way.
Small, tall, metric, imperial — the steps never change.
Common arithmetic mistakes
The most frequent error with the BMI formula is forgetting to square the height, which produces a number several times too high — if your answer looks absurd, check the squaring first. Close behind: feeding centimeters into the meters formula without dividing by 100, and forgetting the 703 in imperial. Every classic mistake is a unit mistake, and every unit mistake is checkable in five seconds.
Wrong answer? Check the squaring, then the units. Always.
What the result means and what it does not
The standard adult bands are under 18.5 (underweight), 18.5–24.9 (healthy), 25–29.9 (overweight), and 30 and above (obese). The BMI formula gives you the number; the bands give it meaning. What the number does not give you is body composition — it can't tell fat from muscle, which is why the same BMI can look very different on two people. Treat the BMI formula as a screening tool, not a verdict.
When the formula misleads
Athletes with substantial muscle mass often land in the overweight band while carrying little body fat — the BMI formula sees their mass, not its makeup. The same caution applies to the elderly with muscle loss, pregnant people, and growing teens. For these groups, pair the BMI formula with waist measurements or professional guidance.
The BMI formula is a starting point. Starting points have edges.
Related BMITool tools and guides
Check your body mass index with the BMITool BMI calculator, and read more helpful articles on the BMI formula, body composition, and healthy weight management.
Key takeaway for What Is the BMI Formula? Calculate BMI Step by Step
The BMI formula is weight divided by height squared — convert height to meters (or total inches), square it, divide the weight. The 703 bridges the unit systems; the bands give the number meaning.
Frequently Asked Questions
What is the BMI formula?
The BMI formula is BMI = weight (kg) / height (m)² in metric units, or BMI = 703 × weight (lb) / height (in)² in imperial units. Weight divided by height squared — that's the entire formula.
How do I calculate BMI step by step?
Convert your height to meters (divide centimeters by 100), square the height, then divide your weight in kilograms by the result. Imperial users: convert height to total inches, square it, and use the 703 constant.
What is the most common BMI formula mistake?
Forgetting to square the height — it produces a number several times too high. After that, the usual suspects are centimeter mix-ups and the forgotten 703 in imperial calculations.
Why doesn't the BMI formula work for athletes?
Because the BMI formula can't tell muscle from fat. Athletes with substantial muscle mass often land in the overweight band while carrying little body fat — waist measurements tell their story better.
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